1. Technical Field
The present invention relates generally to hard disk drives and, more particularly, to a quad burst servo demodulator for recovering the peak amplitudes of hard disk drive servo burst waveforms, and for converting the recovered AC voltage waveforms to DC voltages for subsequent digital processing.
2. Background Art
As known in the art, hard disk drives typically utilize an embedded servo field having a specific pattern for positioning of the read/write head. This pattern, written permanently onto the disk at the time of its manufacture, generally includes four "bursts" of alternating magnets (dibits), with two of the bursts recorded off the center of a track. By reading the relative strengths of the electrical signals produced when the read head of the hard disk drive passes over these bursts, a feedback control loop can be established to position the head exactly over the center of a specific track.
Two systems for reading the relative strengths of the servo burst waveforms are in wide use today: peak detectors and area integrators. Peak detectors quickly acquire and store the peak amplitudes of the waveforms during each burst, usually on discrete internal or external capacitors. Integrating detectors calculate and store the area under the burst voltage verses time curve for each burst.
Examples of existing systems for reading burst waveforms may be found in the following United States Patents, each incorporated herein by reference: U.S. Pat. No. 4,669,004, "High Capacity Disk File with Embedded Sector Servo," (issued May 1987 to Moon et al. and assigned to Quantum Corp.); U.S. Pat. No. 4,698,523, "Servo Data Demodulator," (issued October 1987 to Gershon et al. and assigned to Advanced Micro Devices, Inc.); U.S. Pat. No. 4,879,478, "Fast Sample and Hold Circuit," (issued November 1989 to Gershon and assigned to Advanced Micro Devices, Inc.); U.S. Pat. No. 5,115,360, "Embedded Burst Demodulator and Tracking Error Generation," (issued May 1992 to Sidman and assigned to Digital Equipment Corp.); U.S. Pat. No. 5,276,564, "Programmable Start-of-Sector Pulse Generator for Disk Drive Using Embedded Servo Bursts and Split Data Fields," (issued January 1994 to Hessing et al. and assigned to Hewlett-Packard); and U.S. Pat. No. 5,311,376, "Information Detecting System," (issued May 1994 to Joan et al. and assigned to Western Digital).
Although peak burst waveform detecting circuits are easy to implement, they are overly sensitive to noise; a noise "glitch" occurring at or near the peak of the burst waveform will be stored by the peak detector and give incorrect results. Integrating detectors are much less sensitive to noise, since they do not track and hold peak information, but are more complex to implement.